One of the difficulties in diffusion surface alloying using powdered mixtures is the rapid exhaustion of the saturating mixtures. This can lead to undesirable consequences as the number of cycles using the alloying mixture increases: the repeatability of the alloying results may suffer; the integrity and uniformity of the surface protective layer may deteriorate on workpieces with minor surface defects, such as traces of corrosion or oxidation; the thickness of the diffusion layer may decrease; the concentration of the main alloying element in the diffusion layer may decrease; and the cost of treatment may increase due to the periodic need to replace the exhausted saturating mixture with a new composition. Another disadvantage of existing compositions relates to the waste of a large portion (up to 90%) of the main alloying element present in a saturating mixture that has lost its activity. In addition to the above, the use of existing compositions may lead to lost production time because it is much more difficult to prepare a new saturating mixture than to correct the composition of an existing one before it loses its activity.
There are several types of compositions for diffusion surface alloying and diffusion carbide surface alloying of ferrocarbon workpieces with chromium:
A) Compositions that contain chromium, aluminum oxide, and ammonium halide (typically chloride), for example, chromium 50%, aluminum oxide 43-45%, and ammonium chloride 5-7%.
B) Compositions that contain ferrochromium, aluminum oxide, and ammonium halide, for example, ferrochromium 70%, aluminum oxide 29%, and ammonium chloride 1%.
C) Compositions similar to A) and B) that contain microadditives, such as tantalum, vanadium, molybdenum, boron, silicon, tantalum carbide or silicon carbide, to modify the diffusion layer on workpieces being alloyed. Such microadditives may also act as a reducing agent for the main component of the mixture and for surfaces of workpieces being alloyed. For example, a composition C)(1) contains chromium 50-60%, tantalum carbide 0.75-2.5%, aluminum oxide 34.5-48.25%, and ammonium chloride 1-3%.
D) Compositions that contain chromium oxide, aluminum, aluminum oxide and ammonium chloride, for example, chromium oxide 60%, aluminum 12%, aluminum oxide 25%, and ammonium chloride 3%.
E) Compositions similar to A) and B) but with the addition of a reducing agent such as magnesium to stabilize the alloying results, for example, chromium 65%, aluminum oxide 30%, magnesium 4.5%, and ammonium chloride 0.5%.
F) Compositions that produce mixed chromium-silicon diffusion layers, for example, ferrochromium 25%, aluminum oxide 71%, ferrosilicon 2%, ammonium chloride 1%, and calcium fluoride 1%.
When compositions such as A), B) and C) above are used over multiple cycles (5-10 cycles, sometimes 15-30 cycles), one may periodically add 1-5% ammonium chloride and 20% of the initial composition to replenish the used composition. For example, such an additive or correction composition may be added every fifth working cycle (firing) of the saturating mixture. However, the repeatability of alloying results may still deteriorate as the number of working cycles increases.
As shown in Table 1, the activity of the above composition C)(1), corrected by adding 1-5% ammonium chloride and 20% of the initial composition every fifth working cycle, falls after the tenth working cycle. The fall in activity can be seen from the gradual decline in the surface chromium concentration, a reduction in microhardness and a reduced corrosion potential. The reduced carbon concentration in the carbide layer is believed to be caused by the composition's lost reducing ability in relation to chromium and iron. The activity of compositions such as A) and B) over multiple cycles typically falls even more rapidly.
While a composition such as F) above has the ability to deposit two alloying elements (Cr and Si) simultaneously, which may be beneficial for hot gas corrosion resistance, it is unacceptable for applications where chromium should be the single alloying element in the diffusion layer, for example, marine corrosion applications and abrasion wear applications where a pure chromium carbide layer (such as Cr7C3) provides superior properties compared to a mixed chromium-silicon diffusion layer.
Thus, there is a continuing need for improved compositions and methods for diffusion surface alloying and diffusion carbide surface alloying of ferrocarbon workpieces.